SUMMARY ABSTRACT. Neuroelectric oscillations reflect rhythmic fluctuations of neuronal ensembles between high and low excitability states. These fluctuations clearly impact on sensory processing and cognitive operations, but their actual utility is unclear. Our broad goal is to explore the overarching hypothesis that low frequency neuronal oscillations can function as instruments for active selection of task-relevant inputs in visual cortex and for enhancement of their representation across processing stages. A key proposition of this hypothesis is that in dealing adaptively with ever-changing tasks, ranging from rhythmic (e.g., watching a person walking by) to random (e.g., waiting for a traffic light to change), the brain shifts dynamically between rhythmic and continuous modes of operation. This proposition makes specific predictions vis--vis neuronal oscillations, and these will be explored by paired linear array multielectrode recordings from V1 and extrastriate visual cortices in awake- behaving monkeys. We have 3 specific aims. Aim 1: investigate the mechanistic role of neuronal oscillations in inter-modal attention. Expt 1 will build on our earlier studies on intermodal selective attention (b.2.a; c.2.1). Specific new predictions will be tested using behavioral measures, along with laminar current source density and multiunit activity profiles sampled during task performance using linear array multielectrodes positioned in V1, V4 and inferotemporal (IT) cortex. Inter-areal interactions (prediction 4) will be evaluated using dual multielectrode recordings. Aim 2: evaluate the generality of oscillatory involvement in attentional function and dysfunction. Generalization of finding outside of intermodal selection is important to establish because most selective attention investigations focus on effects within one sensory modality. Thus, Expt 2 will use a visual feature- attention task. To maintain comparability, stimulus manipulations, recording methods and tests of predictions will otherwise generally parallel those of Expt 1. The paradigm in Expt 2 is adapted from studies examining the neurophysiology of attentional deficits in schizophrenia. Expt 2 thus has additional translational value, in that it can determine the physiological significance of ERP components and/or oscillatory characteristics that are shown to be abnormal under corresponding conditions in schizophrenia (b.3; b.5; b.6). Aim 3: investigate the relevance of rhythmic mode processing in natural vision. Expt 3 will use a visual search paradigm, along with a match to sample paradigm, to evaluate the hypotheses that: 1) the key feature of rhythmic mode processing, the rhythmic fluctuation of neuronal excitability, is also a dominant feature in natural visual behavior, and 2) that this is due, at least in part, to the rhythmic nature of visual (saccade/fixation) sampling behavior, and the powerful organizing effects of fixation onset on cortical oscillatory activity (c.2.3). Expt 3 will also examine effects of fixation onset on oscillatory synchrony across cortical layers and across cortical areas.